This invention relates to granulated fertilizer. In a particular aspect, this invention relates to a granulated fertilizer consisting of phosphate and potassium which is agglomerated by use of a slurry.
The manufacture of triple superphosphate by the slurry process is well-known. It has been described in several references. According to J. J. Yarnell, "Phosphorous and Potassium", No. 80, November/December, 1975, published by the British Sulphur Corporation Limited, phosphate rock and phosphoric acid are mixed together in the first of two continuously stirred reactor tanks. The first tank then overflows to the second tank, which is usually larger. As the reaction progresses a slurry is formed, which is then pumped to a granulator where it is sprayed on a rolling bed of recycle material which is partially wetted by the slurry. Soon thereafter, the slurry begins to undergo a phase change from fluid to gelatinous which bonds the recycle particles together into agglomerates. From the granulator, the material is discharged to a dryer where the moisture is removed until only about 2.5% water remains. After screening, oversize is crushed and recycled with undersize to the granulator. The product portion is cooled and then sent to storage.
It has been customary to use high quality phosphate rock of about 72% BPL (Bone Phosphate of Lime, i.e. tricalcium phosphate equivalent) in making triple superphosphate. This is necessary as the desired grade for marketing is 46% available P.sub.2 O.sub.5, and when lower grade rocks are used this guarantee cannot be met. Also, most producers of granular triple superphosphate who use a slurry process, require a finely ground rock, typically 99% minus 60 mesh, 94-96% minus 100 mesh, and 81-83% minus 200 mesh.
Granulation of nitrogen-containing fertilizers (X-X-X grade) is also known. In this process, liquid phase required for agglomeration is obtained by using ammonia or ammoniating solutions to react chemically with phosphate materials or sulfuric acid. The salts thus generated in the hot, wet medium are themselves part of the liquid phase in the ammoniator-granulator equipment.
The ammoniation and neutralization reactions are exothermic, and as the temperature increases, so does the solubility of the entire process mass. Water and steam are also used to optimize the tendency of these liquid-solid combinations to adhere to each other and form agglomerates. However, in O-X-X grades there is no nitrogen, and consequently, the heats of reaction of ammonia with other chemicals are not available to provide liquid phase. Likewise, the highly soluble salts, ammonium-nitrate or urea, cannot be used either. In the O-X-X formulations, only water and steam are available as agglomerating agents.
The production of steam and evaporation of water requires energy. At today's escalating fuel costs, these are not attractive choices as agglomerating agents. Also, water or steam granulation is inefficient in achieving agglomeration, and produces a soft, easily broken particle when dried. There is, therefore, a need for an improved process for producing granulated fertilizer.